This invention relates to the use of ultra-wideband (UWB) radio communication systems. In particular, it pertains to a transmitted-reference, delayed hopped (TR/DH) UWB radio communications system for use in an asset tracking system.
Wireless, narrowband or conventional spread-spectrum communications systems have been proposed for tracking objects within an area of interest. Some of these systems are bi-directional and operate via polling. Others are one-way and transmit based on movement of an asset tag (a small transmitter attached to an item to be tracked) or according to some predetermined schedule which is independent of environmental conditions. Still other systems track objects based on time-difference-of-arrival (TDOA) information or by using crude field-strength measurements. In addition to their tracking function, such systems have been used to provide discrete status information which, for example, may indicate whether a device is powered on.
While narrowband and spread-spectrum systems have proven useful, they are not without drawbacks. For example, the performance of such systems may be adversely affected by interference from strong local RF emitters. The opposite is also true; i.e., because their transmitted energy is concentrated in a relatively limited spectrum, narrowband and conventional spread-spectrum systems may interfere with sensitive communications equipment located within or even external to the operating region.
Narrowband or spread-spectrum systems have also demonstrated poor performance in urban areas, or areas where there is a high concentration of electromagnetic interference. For example, such transmissions are often unable to penetrate building components (e.g., walls, steel structures, elevator shafts, etc.), thereby making their use impracticable in many cases. In addition, narrowband systems often require large power margins in order to combat significant frequency-selective fading, which is associated with indoor RF transmissions.
Time-Difference-of-Arrival (TDOA) is one method that is used to estimate the point of origin of a transmission observed at several receivers. This method requires that each receiver have access to a global clock or timebase so that local Time-of-Arrival (TOA) estimates made at the individual receivers can be compared. An observed difference in TOA at two receivers of known location defines a hyperbola on the plane containing the two receivers and the transmitter. Since the location of the application described here is indoors, the range of values taken by the height of the transmitter off the floor is limited. This being the case, one can approximate the problem by treating all receivers and all asset tags as though they lie in the same plane.
Because the TOA estimates from the various receivers will contain errors, the curves defined by pairwise differences of all receiver measurements will not necessarily intersect. Various procedures can be defined to develop a location estimate from such data. Perhaps the simplest of these is to pick the point on the plane that minimizes the sum of the normal distances from the point to all of the hyperbolas defined by the TDOA measurements. This computation typically takes place at a central computer, which communicates with the RF receivers, typically over a secondary wired network.
The secondary wired network is also used to distribute the global timebase to all the receivers in the system, which requires bandwidth on the wired network as well as extra hardware at the receivers. The requirement of extra bandwidth on the secondary wired network is a drawback, because it contributes substantially to the system cost.
To exemplify, U.S. Pat. No. 5,119,104 notably describes a conventional spread-spectrum system which operates by distributing a clock to a plurality of receivers in a tracking environment, and which then uses that clock to gate a time-of-arrival (TOA) count within each of a plurality of receivers. The TOA count is used to estimate the RF propagation distance between a transmitting tag and the receiver. To maintain ranging accuracy, the system clock must have very little skew between the receivers since each 1 ns of clock skew could introduce as much as 1 foot of ranging error in the system. Clock skew is easily introduced when the system clock is distributed with a cable. Therefore, cable lengths must be carefully measured or controlled during installation of the system, and calibrations must be made in the system to account for the different clock skews introduced by the cabling. This increases system cost and complicates installation as well as maintenance and repair of the system.
In view of the foregoing, it is therefore apparent that there is a need for an RF asset tracking system which overcomes the drawbacks of the conventional narrowband and spread-spectrum systems, and more specifically one which operates more reliably and economically regardless of environmental conditions and without the costly requirement of providing a single clock to all receivers in the system.